Headphone listening is getting more and more popular due to the penetration of portable audio players. Even mobile phones nowadays also allow music playback on headphones. Headphones are also increasingly used in rear-seat entertainment (RSE) systems: this allows people sitting at the back of the car to listen to music or watch DVDs without being disturbed by the music being played on the main car audio installation, and without disturbing the front passengers.
Another key trend is the growing use of Active Noise Reduction (ANR) headphones, which isolates the user from the ambient sound (for instance car/aircraft engine noise, fan noise, train/metro) by means of anti-sound played through the headphone loudspeakers. The anti-sound is calculated from microphones placed on the headphone.
A usual shortcoming encountered while using headphones is the need to respect the left/right order, i.e. ensuring that the left (right) headphone is on the left (right) ear. A left/right inversion may be not dramatic in case of music listening, but for instance in case of movie playback and augmented reality systems (such as auditory displays), a left/right inversion has a negative impact on the overall experience. In both cases, the sound sources played on the headphone indeed relate to a physical location (the screen in case of movie playback, a physical location in case of auditory displays).
Although headphones may be marked with a “L” on the left earpiece and a “R” on the right earpiece, it is not convenient for the user to look for those indications each time the user has to put the headphones on. Some conventions exist though, like cable plug on the left side for full-size headphones or shorter cable on the left side for in-ear headphones, but they are not generalized and do not prevent the user from swapping the channels inadvertently.